1. Technical Field
This is related to time delay devices for optical systems, and more particularly to time delay devices suitable for use in microwave photonics systems.
2. Related Technology
In microwave photonic systems, microwave signals are modulated onto optical carriers, transmitted and subsequently received by photodetectors to recover the amplitude and phase. Optical-domain, radio-frequency (RF) true-time-delay (TTD) lines with programmable time delays, wide bandwidth and low optical loss are key components of microwave photonic signal processing systems and future optical communications networks.
Their unique advantages, including low loss (independent of microwave frequency), large instantaneous bandwidth, immunity to electromagnetic interference, and parallel signal processing capability, have led to the realization of high-performance, tunable microwave filters, phased array beamformers, fast analog-to-digital converters, arbitrary waveform generators, signal correlators, and frequency converters and mixers.
For such applications, it is important that the delay lines exhibit low loss, wide RF bandwidth, minimal frequency-dependent loss and dispersion, as well as achievable time delays at least on the order of the RF period. In addition, rapid tuning speed and fine delay resolution are of interest for enabling higher-performance, more agile photonic systems.
A. J. Seeds and K. J. Williams, “Microwave photonics,” Journal of Lightwave Technology, Vol. 24, pp. 4628-4642 (December 2006), describes the history and development of the field of microwave photonics. R. A. Minasian, “Photonic signal processing of microwave signals,” IEEE Transactions on Microwave Theory and Techniques, Vol. 54, pp. 832-846 (February 2006) discloses photonic signal processing techniques using optical delay lines for processing high bandwidth signals. R. S. Tucker, “The role of optics and electronics in high-capacity routers,” Journal of Lightwave Technology, Vol. 26, pp. 4655-4673 (December 2006), discloses the potential for using optical delay lines as buffers for routers.
Although optical-domain, RF TTD lines have been demonstrated, relatively few are currently capable of fine-resolution tuning over an entire RF period at 10 GHz and above. Discrete fiber Bragg grating (FBG) delay lines are discussed in R. A. Minasian, “Photonic signal processing of microwave signals,” IEEE Transactions on Microwave Theory and Techniques, Vol. 54, pp. 832-846 (February 2006). The discrete FBG delay lines, however, are limited to minimum delay increments of 10 ps, which translates to a coarse 36° RF phase step at 10 GHz.
K.-L. Deng, I. Glask, P. Prucnal, and K. I. Kang, “A 1024-channel fast tunable delay line for ultrafast all-optical TDM networks,” IEEE Photon. Technol. Lett. Vol. 9, No. 11, pp. 1496-1498 (1997) discloses integrated-optic switch delay lines. These integrated-optic switch delay lines, on the other hand, can suffer from prohibitive optical loss at large bit-depths, and can be challenging to phase-trim to 1 ps.
D. A. Henderson, C. Hoffman, R. Culhane, D. Viggiano III, M. A. Marcus, B. Culsahw, and J. P. Dakin, “Kilohertz scanning, all-fiber optical delay line using piezoelectric actuation,” Proc. SPIE vol. 5589, pp. 99-106 (2004) discloses piezoelectric fiber stretcher delay lines. These Piezoelectric fiber stretchers can provide both fine delay resolution and low optical loss, but are currently limited to tuning ranges on the order of 10 ps, over which the tuning can be slow.
Delay systems using chirped fiber gratings are disclosed in D. B. Hunter, M. E. Parker, and J. L. Dexter, “Demonstration of a continuously variable true-time delay beamformer using a multichannel chirped fiber grating”, IEEE Trans. Microw. Theory Tech., Vol. 54, No. 2, pp. 861-867 (2006), and in B. Zhou, X. Zheng, X. Yu, H. Zhang, Y. Guo, and B. Zhou, “Impact of group delay ripples of chirped fiber grating on optical beamforming networks,” Opt. Express, Vol. 16, No. 4, pp. 2398-2404 (2008). However, delay lines based on chirped FBGs can suffer from group delay ripple (non-linear variations in group delay across the optical bandwidth) at high frequencies, typically on the order of 10 ps, which limits the achievable phase and amplitude control.
Other approaches for finely-tunable TTD at 10 GHz include linearly-translated mirror, 3D linear switch array and highly-dispersive fiber delay lines.
3D linear switch arrays are disclosed in V. Kaman, R. J. Xuezhe Zheng, C. Helkey, C. Pusarla, and J. E. Bowers, “A 32-element 8-bit photonic true-time-delay system based on a 288×288 3-D MEMS optical switch,” IEEE Photon. Technol. Lett., Vol. 15, No. 6, pp. 849-851 (2003). Highly-dispersive fiber delay lines are disclosed in M. Y. Frankel, P. J. Matthews, and R. D. Esman, “Fiber-optic true time steering of an ultrawide-band receive array,” IEEE Trans. Microw. Theory Tech., Vol. 45, No. 8, pp. 1522-1526 (1997). The fiber delay lines can provide a continuously-tunable time delay via either a tunable laser, as described by Frankel et. al, by via a nonlinear optical wavelength conversion process, described in N. Alic, J. R. Windmiller, J. B. Coles, and S. Radic, “Two-pump parametric optical delays,” IEEE J. Sel. Top. Quantum Electron. 14(3), 681-690 (2008).
The 3D linear switch approach can be expensive, however, because at least N switchable, phase-trimmed paths are required to realize N programmable delay levels (a linear architecture is necessitated by the small incremental delay). Translated mirrors, on the other hand, are relatively slow. Dispersive fiber delay lines can suffer from poor tuning speed when using high-performance, tunable distributed-feedback (DFB) lasers, because temperature stabilization within a small fraction of the full temperature tuning range is rather slow. Rapidly-tunable lasers can be substituted to provide faster tuning, but this comes at the expense of increased laser noise and reduced power, which ultimately impacts system performance.
Scanning delay lines developed for optical coherence tomography (OCT) are described in D. Piao and Q. Zhu, “Power-efficient grating-based scanning optical delay line: time-domain configuration,” Electronics Letters, Vol. 40, pp. 97-98 (2004), and in Z. Jiang, Q. Zhu and D. Piao, “Minimization of geometric beam broadening in a grating-based time-domain delay line for optical coherence tomography application,” J. Opt. Soc. Am. A, Vol. 24, pp. 3808-3818 (2007).
There is continued interest in developing optical TTD delay lines capable of rapid tuning and fine delay resolution at 10 GHz and beyond.